1. Field of the Invention
The present invention relates to a process for preparing low viscosity fatty acid-modified polyesters of uniform molecule structure for use in baking varnish systems of low solvent content.
2. Description of the Prior Art
Most baking varnish systems employed in practice are derived from fatty acid-modified polyesters (alkyd resins) and amino, phenolic, and acrylic resins or polyisocyanates. These baking varnish systems generally have a solids content of from about 50 to 55 percent, the balance being organic solvents. Baking these varnishes requires considerable amounts of heat in the evaporation step. Expensive technical equipment must be provided and substantial processing costs are incurred when recovering, or disposing of the solvents because they cannot, with respect to environmental protection, be simply expelled into the open air. For these reasons every effort has been made in the past to eliminate the organic solvents completely, or at least partially. For instance, water-soluble alkyd resins were developed. These alkyd resins, however, suffer from a serious disadvantage in that they require amines detrimental to environmental in the neutralization step and organic solvents, as well, and they are difficult to process. Attempts have also been made to provide a baking varnish system free from solvent which resulted in powder resins. The technique of applying thin layers, of obtaining the desired colour shades as well as changing to other hues, when employing these lacquer resin films still presents difficulties so that, even today, they are used in only a limited field of application. A further disadvantage when employing powder resins, is the very high cost of investment for the powder spray coating means. Today attempts are being made to utilize the so-called low solvent baking varnish systems, i.e., systems having low solvent content. Such varnish systems are processed in the same manner as the conventionally used, solvent-rich lacquer systems. In such low solvent systems which contain from 70 to 80 percent less solvent than the conventional solvent-rich lacquer systems, low viscosity fatty acid-modified polyesters (alkyd resins) having molecular weights from 800 to 2000 are employed. Such fatty acid-modified polyesters are described in German Offenlegungsschrift Nos. 2 253 325 and 2 019 282. The alkyd resins employed in the past have high viscosities and thus require large amounts of solvent because the range of the molecular weights is very broad as a result of the equilibrium conditions encountered in the production of the alkyd resins by esterification reactions. The molecular sizes range from ester monomers to materials which are slightly gelated. Attempts have been made to narrow the range of the molecular weights of the esters by varying charge materials, amounts and esterification conditions but because of the adjusting of the equilibrium in the esterification reactions, fatty acid-modified polyester molecules of the same molecular weights cannot be obtained. One of the problems associated with the preparation of alkyd resins according to the aforementioned Offenlegungsschriften and according to Offenlegungsschrift Nos. 2 504 550 and 2 410 512 is that they are prepared from oligomer mixtures derived from molecules of various chain lengths and structures as well as being dependent on the selected mole ratio of polyhydric alcohol to polycarboxylic acid or monocarboxylic acid and on the grade of esterification, as well, and the finished resins contain relatively high amounts of non-reacted poly-alcohol and of monocarboxylic or dicarboxylic acids which adversely affect the resistance to scraping, to chemicals, to solvents, and to weather, of the lacquer films produced thereby.
It is the object of the invention to provide a process for preparing low viscosity fatty acid-modified polyesters (alkyd resins) with all molecules being substantially of the same structure and of the same chain length in order to obtain, e.g., in combination with amino resins, phenolic resins, acrylic resins, or polyisocyanates, low solvent baking varnishes of improved properties.